This invention relates generally to analog/digital hybrid control loops, and more specifically, to a system for interfacing a digital computer with at least one analog control loop to increase the agility of the analog loop without imposing unreasonable data rate requirements on the computer.
A representative analog/digital hybrid control loop is shown in the functional block diagram schematic of FIG. 1. This particular control loop includes a rate-integrating gyro within a gimbaled system for controlling a first or inner gimbaled element and a second or outer gimbaled element. While the schematic diagram illustrates the control of motion in only one dimension, like the azimuth or elevation dimension, for example, it is understood that the digital computer may control other dimensions of the gimbaled system in a similar manner.
Referring to FIG. 1, a digital computer 12 is operative as an angular position controller by setting a desired angular position .theta..sub.s for the gimbaled system and subtracting therefrom in the summer 14 the measured angular position .theta..sub.f of the gimbaled system 10. The resulting positional error .theta..sub..epsilon. is operated on by a conventional proportional plus integral (P+I) controller 16 to generate a rate command signal 18 which is provided to the gimbaled system 10 via a conventional digital-to-analog converter (D/A). The analog rate command signal 20 may be provided to a rate-integrating gyro and driving circuits 22. More specifically, the analog signal 20 governs the gyro precession rate (i.e., rotation of the spin axis of the gyro) by gyro torquing circuits 24 and causes the first or inner gimbaled element to move at the same angular rate therewith. The angular rate of the gyro/inner gimbaled element is denoted functionally by the signal .theta..sub.GY provided to the functional summer 26.
A gimbal torque motor, denoted by the block 28, may be powered by an amplifier 30 to drive a second or outer gimbaled element coupled thereto at an angular rate which is functionally denoted by the signal line .theta..sub.LOS also provided to the summer 26 to be functionally subtracted from the angular rate of the inner gimbaled element in the gyro unit 22. A functional signal 32 representative of the angular rate difference thereof is functionally integrated and monitored by a sensor element 34 of the gyro unit 22 to effect the positional difference or displacement between the inner and outer gimbaled elements during movement thereof. An angular displacement signal 36 may be generated by the sensor 34 and provided to a number of signal conditioning circuits 38 for various functional operations like demodulation and filtering, for example. A resulting conditioned angular displacement signal 40 may be passed through conventional compensation networks 42 and 44 to govern the angular rate movement of the outer gimbaled element via power amplifier 30 and torque motor 28. In the present example, the angular position of the outer gimbaled element is used as the positional control element for the digital computer 12. For this case, a functional integration of the angular rate .theta..sub.LOS may be accomplished by a conventional instrument 48, like a synchro, for example, to effect a positional signal 50 which is digitized by a conventional analog-to-digital converter (A/D) and provided to the functional summer 14 via digital computer 12.
In operation, the computer 12 may govern the movement of the outer gimbaled element to a new position by changing the set point .theta..sub.s of its positional loop. Upon changing the set point .theta..sub.s, an error .theta..sub..epsilon. is created and is operated on by the P+I controller 16 to produce a rate command signal 18 which is passed to the gyro torquer 24 via D/A converter. An angular rate is caused to occur in the gyro/first gimbaled element which is sensed as an angular rate difference 32 by the gyro unit 22. The gyro sensor 34 integrates the angular rate difference to effect a signal 36 representative of the angular displacement between the first and second gimbaled elements. The signal 36 is utilized to drive the torque motor 28 to cause the second gimbaled element to follow the first gimbaled element at the desired angular rate thereof. As the position of the second gimbaled element approaches its desired position, the positional error .theta..sub..epsilon. converges to zero, causing the rate command signal to stabilize. The angular rate control loop maintains the desired angular position until a new angular position is effected by the digital computer 12.
One drawback of analog/digital hybrid control loops of this variety is that the computer is a quasi-synchronous machine and thus updates the rate command signal stepwise at designated time intervals. Because of the stepwise manner of generation of the rate command signal, the analog control loop may become saturated by abrupt and frequent changes in the rate command signal governing it. More specifically, as the rate command signal from the computer changes abruptly, the spin axis of the gyro or inner gimbaled element quickly begins rotating at a newly commanded rate .theta..sub.GY. The resulting loop error signal 32 eventually creates an acceleration signal to produce the desired rate change in the second gimbaled element. However, before that can happen, the error signal 32 may become so large as to produce saturation within the rate stabilization loop. The reason for this is that the loop is made very "stiff", in order to provide good outer gimbaled element stabilization in spite of such things as torque disturbances caused by gimbaled element friction, cable torques, and the like. This high stiffness design may cause the torque demand to exceed the capability of the gimbal torque motor 28 when the position error or displacement 36 is still quite small.
To keep such saturation from causing instability of the loop 10, some torque motor analog control loops include a gyro saturation loop, like that shown at 54, for example. Such a saturation loop becomes operative when the torque limit or angular displacement between the first and second gimbaled elements exceed a predetermined level. In the present example, the saturation loop 54 includes a non-linear functional element 55 which has as an input the conditioned angular displacement signal 40 and an output which couples to a compensation network 58 to provide a signal 60 which is used to further govern the gyro torquer 24. The output signal 60 has no effect on the gyro unit 22 as long as the angular displacement signal 40 is within predetermined upper and lower limits 56 and 57, respectively, of the functional unit 55. Should the signal 40 exceed the displacement limits, the functional unit 55 governs the gyro torquer 24 via the compensation network 58 with a signal value in proportion to the amount of displacement exceeding the predetermined limit.
These type of saturation loops, like that shown at 54, for example, are needed to prevent instability when the stabilization loop becomes non-linear, because of torque limiting, for example. Unfortunately, the saturation loop 54 changes the gyro/inner gimbaled element position (i.e., during saturation) in a manner not in accordance with the computer generated rate command signal 20 and thus, corrupts the inertial reference for the second gimbaled element angular position. The present invention which is described hereinbelow intends to avoid such corruption of the reference position of the second gimbaled element during periods of rapid change in the desired gimbaled element rate, without sacrificing agility or accuracy of the actual rate.
It is apparent that the abruptness of rate command signal changes could be reduced by analog low-pass filtering of the computer generated rate command signal, but that would create a time lag in the analog control loop input and be counter-productive in terms of analog loop agility. For this reason, an analog filtering element in and of itself is counter to the purpose of increasing the agility of the analog control loop without imposing unreasonable data rate requirements on the computer. However, a carefully blended combination of elements including an analog filtering element may result in the preferred operation if the average loop output during the time interval between computer generated rate command signal updates may be controlled to be substantially that desired, even during periods of rapid change in the analog loop output.